Whisker-reinforced alumina has found use in structural applications such as heat engines and turbine blades (see, U.S. Pat. No. 4,543,345) and in more severe applications such as cutting tools (see, U.S. Pat. Nos. 4,789,277 and 4,961,757). These composites can have up to about 40 vol. % whiskers using a processing technique known as "hot pressing" which produces composites having a density of at least greater than 98% of theoretical. A density of at least about 95% is needed for virtually all engineering applications of ceramics.
In the conventional processes for making whisker-reinforced alumina bodies, hot pressing can also be described as "pressurized sintering" because the process involves the application of uniaxial loads of about 3500-5500 psi. In the process, a powder blend or preformed sample is loaded in a free flowing form into a graphite die and sintered under a controlled atmosphere in a high temperature furnace. Pressure is applied to the powder in the die with a hydraulic ram as the furnace temperature is increased to about 1450.degree.-1850.degree. C. and maintained at temperature for an appropriate time. The furnace is then allowed to cool. Cycle times of several hours are used, and the process is limited to the production of discrete batches.
The structural limits of the dies generally prohibit the use of applied pressures greater than about 5500 psi. In addition, the hot pressing method generally limits the shape of the formed body to simple geometric shapes such as round or rectangular plates or cylinders unless special complex dies and pressure rams are constructed at great expense. In addition, the properties of such hot pressed bodies are more anisotropic due to the uniaxial loading than those prepared by the pressureless sintering.
Hot pressing is to be contrasted with "green" preform body preparation, "pressureless sintering" and "hot isostatic pressing" (HIPing). Green preform bodies are prepared by mixing ceramic powder blends with conventional sintering aids (e.g., magnesia and/or rare earth oxides) and an organic binder containing a lubricant. The green preform body is then formed by compacting the powder blend at ambient temperature to a density of about 60-70% of theoretical. The theoretical density of a material is generally measured tycnometrically (ASTM C329-75) or estimated using the accepted "Rule of Mixtures."
Generally, green bodies are formed as simple geometrical shapes (e.g., a tube, cylinder, or disk) and, if necessary, machined to the desired shape allowing for shrinkage which will occur during subsequent heating steps (i.e., near-net-shape fabrication). Binder material is removed by heating the shaped green body in air in an oven at temperatures up to 500.degree. C. for about 10-15 hours depending on size and shape.
A green preform body is further densified by pressureless sintering in a furnace at 1250.degree.-1850.degree. C. under an atmosphere (e.g., argon or nitrogen) that does not adversely affect either the composite being sintered or the sintering furnace components. The sintering is allowed to continue until essentially complete (generally greater than 94% to 95% theoretical density). If the resulting sintered body forms a closed cell structure (closed porosity), the density may be further increased by hot isostatic pressing.
Hot isostatic pressing (HIPing) is the process of applying high pressure to a sintered body with inert gas typically at 15,000-30,000 psi for about 1-2 hours at a temperature of from about 1500.degree. to about 1700.degree. C. (for aluminum oxide) with the goal of producing a body having greater than about 98% theoretical density. The sintered body to be HIPed must exhibit an almost completely closed cell structure for HIPing to have a significant effect. The concentration of closed pore structure is calculated from the formula: EQU (% Closed)=100-(% Theoretical density)-(% Open Porosity)
The method used for measuring the closed porosity is ASTM C830-83. Briefly described, the open pore structure is determined by measuring water uptake during vacuum impregnation of the body. As an example, if a dry sintered body with a density of 95% theoretical density has a water uptake corresponding to 1% of the alumina composite density, then the body has a 4% closed pore structure. The theoretical density is calculated by applying the rule of mixtures to the absolute densities of each component phase, as is well known in the art.
Economically, hot pressing is an extremely expensive and labor intensive process. The pressing equipment is costly. The graphite dies must be cut from a unitary block and do not last for many pressings. The cycle times are relatively short, but can only produce a limited number of specimens. The pressed bodies are limited to simple geometric shapes. The machining of such shapes to final form is difficult due to the high density and hardness of the body and can lead to rejects or surface stresses that affect the structural performance of the part. See, Sacks U.S. Pat. No. 5,009,822.
By contrast, pressureless sintering is about 25-33% the cost of hot pressing. Conducting the process at about atmospheric pressure reduces the capital expense of the equipment needed for the process. Continuous processing can be used, and large numbers of parts can be made. If a batch process is used, the batches are larger and the furnaces can be used more economically. Metal dies used for green forming can be reused for many pieces. The ability to readily machine the green body permits the formation of complex shapes. If needed, HIPing can be used after pressureless sintering to increase the final density of the body at a total cost that is still less than the corresponding hot pressing process (i.e., perhaps 30% less) but without the inherent limits imposed by hot pressing.
Pressureless sintering would, therefore, be a preferred method for making ceramic bodies and for whisker reinforced ceramics in particular. Unfortunately, those in the art have found that pressureless sintering does not produce adequate densities when the composite contains more than 10 vol. % (about 8.1 wt %) whiskers. See, Tiegs U.S. Pat. No. 4,652,413 where an alumina matrix containing 2% yttria as a sintering aid and 10 vol % silicon carbide whiskers was sintered by pressureless sintering to greater than 94% theoretical, but an equivalent sample with 20 vol % (about 16.7 wt. %) whiskers could achieve only 75% theoretical density. This limitation of about 10 vol % whiskers is also described in Sacks U.S. Pat. No. 5,009,822 in col. 7, lines 4-12. Somewhat similarly, WO 86/05480 exemplified sintered densities of greater than about 95% theoretical only for composites containing up to about 12.1 volume % (10 wt %) whiskers (Table I).
A review of the problems associated with pressureless sintering of alumina-SiC whisker composites is presented in Tiegs et al., A. Ceram. Soc. Bull., 66(2) 339-342 (1977). As described on page 340, the whiskers interfere with efficient particle packing, particle rearrangement, and shrinkage. The result is a low final density. Table II and FIG. 2 in Tiegs show that as the whisker content increases, the green and final densities decrease. Tiegs et al. states: "At whisker contents much greater than 10 vol %, the inhibition of densification is acute." Moreover, HIPing was not able to increase the density of the body because "the material had not achieved closed porosity prior to HIPing." The fracture toughness for the 20 vol % whisker material was reported by Tiegs as not significantly higher than monolithic alumina.
In a later paper, Tiegs et al. achieved a 95% density with pressureless sintering of an alumina composite containing about 13 vol % (about 10.7 wt %) whiskers. Ceram Engr. and Sci. Proc., Sept.-Oct. 1986, pp. 1182-1186, FIG. 2. The paper also summarizes the need in the art: "Further development may make it possible to sinter and then HIP (without encapsulation) alumina with up to 20 vol. % SiC whiskers, but that is yet to be shown."
The art has gone to great lengths to find a process that would produce a whisker-reinforced alumina composite with a density of at least about 95% theoretical. Techniques used to increase density include dry processing of powders with pressureless sintering and HIPing (see, Tiegs et al., Ceram. Engr. and Sci. Proc., 13th Automotive Conf., pp. 1182-1186 (Sept.-Oct. 1986)) and wet processing with size classification to remove agglomerates followed by slip and centrifugal casting of well dispersed suspensions of alumina and whiskers (see, Sacks et al., J. Am. Ceram. Soc., 71(5) 370-379 (1988)). These wet processing methods are considered to produce superior green bodies compared to dry powder processing. See, Sacks et al. in Table III. Wet processing techniques can be used to produce green bodies having high densities with even 30 vol % whiskers because green body density is not significantly affected by whisker size and concentration. Compare Sacks et al., J. Am. Ceram. Soc., 71(5) 370-379 (1988) with Tiegs et al., Ceram Engr. and Sci. Proc., (Sept.-Oct. 1986) in FIG. 2. Upon sintering, however, the whiskers exert a controlling influence over the densities which can be attained by sintering green bodies made by either dry or wet processing.
Whisker size is usually discussed in terms of the "aspect ratio", a dimensionless ratio of whisker length to diameter. The generally accepted wisdom in the art is that an aspect ratio of at least 10 is required to reinforce ceramics. See, Milewski et al., Handbook of Reinforcements for Plastics, p. 216-217 (1987) and Tiegs et al. U.S. Pat. No. 5,207,958.
The need continues to exist for a pressureless sintering process that will permit the use of high levels of whiskers as reinforcing agents for alumina matrices and result in a sufficiently high density with a sufficient degree of pore closure to permit HIPing the body to further increase both the density and strength of the ceramic to the level required by the intended application.
One skilled in the art will recognize that the application and purpose for which the reinforced ceramic is to be used will dictate the minimum density and strength that is required. Different applications and end uses will require different minimum densities and strengths. For example, a metal cutting tool insert with 15 wt. % whiskers requires a density of greater than 98% theoretical. Depending on the composition of the ceramic, its application, and end use, HIPing after pressureless sintering may or may not be required. The identification of the requirements for each use is within the skill of the art.
The art has also suggested the use of high levels, e.g. greater than 8 wt %, of sintering aids to increase the composite density. See, Tiegs et al. U.S. Pat. No. 5,207,958. The resulting liquid phase may, however, affect the composite's high temperature properties. Sacks et al., Ceram. Engr. Soc. Proc., 9 [7-8], pp. 741-754 (1988). It would be desirable to have a process that could produce high densities without the need for high levels of sintering aids which adversely affect properties of the composite.
In addition to a high density and closed pores, the final composite should exhibit high levels of fracture toughness. One method for increasing the toughness of a reinforced alumina composite is described in Landingham U.S. Pat. No. 4,745,091. The disclosed method includes hot pressing an alumina composite containing reinforcing particles, magnesia (as a sintering aid), zirconia or hafnia (for toughness and shock resistance), and 0.1-15 wt % of a nitride modifier. Reaction hot pressing is preferred to avoid the need for a preliminary step to pre-react the powders so as to form a second nitride phase (identified as "SiAlOX") uniformly dispersed in the matrix. See, col. 3, lines 6-9; col. 4, lines 30-38; and FIG. 3. If the powders are pre-reacted, the patent discloses that cold pressing and sintering can be used. The examples illustrate the use of a total nitride modifier concentration of 9-30 wt % with hot pressing fabrication. The problem of achieving high compaction densities with high whisker concentrations is not disclosed, acknowledged, nor exemplified.
It would be desirable to provide a pressureless sintering process that could produce whisker-reinforced alumina composites exhibiting levels of toughness comparable in use to those of reinforced alumina composites made by hot pressing.